Based on Nose to the Mindstone which is in turn based on Disk Crash. I like how this card has some versatility as either a straight up mill spell, or a way to call out what your opponent has for casting costs in their deck and try to smash them for getting it right. The higher the casting cost of the card you're asking for, the less likely they will flip it, but you get more chances to hit it...
This card has the added bonus of doing absolutely nothing when X=0.

For flavour reasons I would suggest to let the player lose X life instead of 7.

Well, if you did that, then it wouldn't deal damage when you payed . I mean, maybe your opponent has a card that costs in his deck. I think I wanted to guarantee interaction rather than make a narrow card that only worked right if your opponent was packing enough cards that cost 5 or more in the deck.

That wouldn't change anything about if it deals damage, just how many. Let's say you play it with (3)(b) and it mills a CMC3 card then now it would deal 7 damage, with my suggestion just 3. Play it for (15)(b) (and hit something that expansive, like Emrakul, the Aeons Torn) now: 7 dmg, new: 15 dmg...

I just think I should mention here that "Mind Grind" (two words) is an actual card.